Cardiovascular disease (CVD) remains a leading cause of disability and death in the United States, despite great strides in its treatment over the past decades. Coronary artery disease, the major form of CVD, is the major cause of death in the United States today, responsible for over 550,000 deaths per year. Cerebrovascular disease is the third leading cause of death. The etiology of both coronary artery and cerebrovascular diseases is attributed to atherosclerosis. Through its clinical manifestations, atherosclerosis is the major cause of the more than one million heart attacks and approximately 400,000 strokes that occur each year. In addition to the high morbidity and mortality associated with atherosclerosis, it has been estimated that atherosclerosis has cost the United States' economy over $80 billion each year in lost wages, lost productivity, and medical care costs [Levy, R., Am. Heart J. 110: 1116 (1985)].
Atherosclerosis is the most important form of arteriosclerosis, involving the aorta and all of its branches. The aorta or a major artery may be involved severely with scattered and confluent atheromas in short segments next to perfectly normal vessel wall areas. The atheroma (atherosclerotic plaque) develops slowly over a period of years, finally becoming masses of lipids, mostly cholesterol and cholesterol esters. These interarterial masses reach sufficient size in the lumen of the blood vessel to obstruct blood flow, causing turbulence which may disrupt the surface of the atheroma forming a lesion and micro-emboli. Acute symptoms develop with obstruction of blood flow and as a result of the location of lodgement sites of the micro-emboli.
In the chain of events leading to atherosclerosis, it is believed that the initiating event is the formation of "fatty streaks" in carotid, coronary, and cerebral arteries, and in the aorta. These lesions are comprised of fatty deposits of cholesterol and cholesteryl ester, principally found within the smooth muscle cells and macrophages of the intimal layer [Ross, R., New Eng. J. Med. 295:369 (1977)]. Desquamation or injury of the endothelium, resulting in exposure of and possible disruption of the integrity of the extracellular matrix surrounding the cells, leads to 1) recruitment of circulating monocytes and their differentiation to macrophages, 2) accumulation of lipid in macrophages and smooth muscle cells [Ross, R., New Eng. J. Med. 314: 488 (1986)], 3) a shift in smooth muscle phenotype from a quiescent, contractile state to a migrating, proliferative form [Manderson, J. A., Arteriosclerosis 9: 289 (1989)], 4) eventual migration of transformed smooth muscle cells from the medial layer to the sub-lesion intimal layer [Clowes, A. W., Circ. Res. 56: 139 (1985)] and 5) subsequent massive proliferation of the intimal smooth muscle layer resulting in arterial luminal blockage [Clowes, A. W., J. Cardiovas. Pharm. 14 (Suppl 6): S12 (1989)].
Several risk factors have been identified in individuals who develop atherosclerosis. It can be inferred that persons with at least one risk factor will be at greater risk of developing atherosclerosis than persons with no risk factors. Persons with multiple risk factors are even more susceptible. The risk factors include hyperlipidemia (hypercholesterolemia and/or hypertriglyceridemia), hyperglycemia, diabetes, hypertension, obesity, cigarette smoking, familial hyperlipoproteinemia, aging and male sex. Peri- and post-menopausal women are one particular group of aging persons at risk for developing coronary heart disease. Since the 1950s, it has been observed that premenopausal women are protected from coronary heart disease. These observations prompted several animal studies which demonstrated that the administration of estrogens to animals fed a high fat diet prevented dietary-induced coronary atherosclerosis. [Barren-Connor, E., JAMA 265: (1991)]. One of the mechanisms by which estrogen is thought to be protective against atherosclerotic coronary heart disease is by lowering total plasma cholesterol (TPC) through induction of increased catabolism and excretion of low density lipoprotein (LDL) cholesterol into bile by the liver. This increased LDL catabolism and cholesterol excretion may be a result of an estrogen dependent increase in low density lipoprotein receptors in the liver, as has been demonstrated in rats given large pharmacologic doses of 17.alpha.-ethinyl estradiol. [Chao, Y.-S., J. Biol. Chem. 254: 11360 (1979); Kovanen, P. T., J. Biol. Chem. 254: 11367 (1979); Windler E. E. T., J. Biol. Chem., 255: 10464 (1980)]. Women who receive postmenopausal estrogen replacement therapy (ERT) have been shown to benefit from a fifty to seventy percent reduction in risk from atherosclerotic related coronary heart disease. [Stampfer, M. L., N. Engl. J. Med. 313: 1044 (1985)]. The mortality from CVD is 63% lower and the rate of mortality from myocardial infarction is between 2.3 and 2.7 times lower in estrogen-treated women compared with untreated climacteric women.
While the benefits of ERT in postmenopausal women are substantial, an association has been established between the use of unopposed ERT (estrogen therapy without concomitant progestin administration) and endometrial hyperplasia, thereby increasing the risk of endometrial carcinoma. [Utian, W. H., Obstet. Gynecol. Surv. 32: 193 (1977); ACOG Technical Bulletin 93: 1 (1986); Hammond, C. B., Fertil. Steril. 37: 5 (1982); Whitehead, M. I., Am. J. Obstet. Gynecol. 142: 791 (1982); Gambrell, R. D., South Med. J. 71: 1280 (1978); McDonald T. W., Am. J. Obstet. Gynecol. 127: 572 (1977)]. To reduce or entirely eliminate the risk of endometrial adenocarcinoma resulting from ERT while maintaining the benefits of ERT, it has been shown that progestins can be administered concomitant with the estrogen during the last 10-14 days of each estrogen cycle. [Whitehead, M. I., J.R. Soc. Med. 72: 322 (1979); Whitehead, M. I., Semin. Reprod. Endocrin. 1: 41 (1983); Barrett-Connor, E., Annu. Rev. Med. 43: 239 (1992)].
17 alpha-Dihydroequilin sulfate is approximately 15% by weight component of Premarin.RTM. conjugated estrogens (Premarin is a Registered Trademark of Wyeth-Ayerst), a drug commonly prescribed as ERT in postmenopausal women. There have been several reports of the relative estrogenicity of various estrogens to determine if they have differential effects on menopausal vasomotor symptoms, urinary gonadotropin levels, plasma lipid and lipoprotein metabolism and hepatic globulin synthesis compared to uterine response. Two studies showed that 17.alpha.-dihydroequilin had little stimulatory effect on uterine weight or maturation of vaginal cytology in the rat model and did not suppress urinary gonadotropins in postmenopausal women. Estrogenic activity was minimal. [Howard, et al., Arch Int. Med. 128: 229 (1971); Stern, Maturitas 4: 333 (1982)]. Neither study suggested the use of 17 alpha-dihydroequilin in reducing atherosclerosis.
The treatment of atherosclerosis is generally directed toward attenuation of sequelae (angina pectoris, myocardial infarction, arrhythmias, heart failure, kidney failure stroke, peripheral arterial occlusion, and related disease states) with administration of antilipidemic drugs, reduction of blood pressure by 10 to 20% and increasing high density lipid blood levels by diet and exercise. These measures are generally designed to slow the rate of progress of the disease state rather than reverse its direction.